In general, a motor vehicle automatic transmission includes a number of gear elements and selectively engageable friction elements (referred to herein as clutches) that are controlled to establish one of several forward speed ratios between the transmission input and output shafts. The input shaft is coupled to the vehicle engine through a fluid coupling such as a torque converter, and the output shaft is coupled to the vehicle drive wheels through a differential gearset.
Shifting from a currently established speed ratio to new speed ratio involves, in most cases, disengaging a clutch (off-going clutch) associated with the current speed ratio and engaging a clutch (on-coming clutch) associated with the new speed ratio. Each such shift includes a fill or preparation phase during which an apply chamber of the on-coming clutch is filled in preparation for torque transmission. Once filled, the on-coming clutch transmits torque in relation to the clutch pressure, and the shift can be completed using various control strategies.
Typically, the fill phase is carried out by commanding a maximum on-coming clutch pressure for an empirically determined fill time. See, for example, the U.S. Pat. No. 4,707,789 to Downs et al., issued on Nov. 17, 1987, and assigned to the assignee of the present invention. In Downs et al., the fill time for any given clutch is obtained from a look-up table as a function of the commanded fill pressure, less the clutch return spring pressure, and then adjusted as a function of fluid temperature to compensate for variations in fluid viscosity. In operation, the stored fill times are then adaptively adjusted based on the observed inertia phase delay, in order to compensate for sources of error that influence the fill time. A similar approach is found in the U.S. Pat. No. 5,216,606 to Lentz et al., issued on Jun. 1, 1993, and assigned to the assignee of the present invention. In Lentz et al., the fill time is determined by table look-up based on the pump speed, as compensated for efficiency and fluid viscosity, and further modified based on the time since the last shift and a hydraulic restriction factor.